Wet Runway and its Effects on V1 in a large aircraft

[Alchemy]

Hi,

Well, the airplane I fly now is not particularly large so V1 is not appreciably effected by contaminated runways (V1 and Vr are almost always the same, basically centerline thrust, brakes and tire speed no factor). However, I've recently encountered some questions about such a scenario.

Does water on the runway result in increased V1 due to an increased accelerate-go distance?

Or does water on the runway result in decreased V1 due to an increased accelerate-stop distance?

I've read some conflicting resources and can't seem to come up with a straight answer. Any help and insight is appreciated.

My best answer is that V1 should decrease on a WET runway, because the accelerate stop distance decreases to a greater degree than the accelerate go distance (deceleration is a greater problem than acceleration). On runways contaminated with snow or slush, V1 should increase because the inverse is true, acceleration is hindered more than deceleration.

Does this seem accurate?

Thanks!

 

[jynxyjoe]

Hi,

Well, the airplane I fly now is not particularly large so V1 is not appreciably effected by contaminated runways (V1 and Vr are almost always the same, basically centerline thrust, brakes and tire speed no factor). However, I've recently encountered some questions about such a scenario.

Does water on the runway result in increased V1 due to an increased accelerate-go distance?

Or does water on the runway result in decreased V1 due to an increased accelerate-stop distance?

I've read some conflicting resources and can't seem to come up with a straight answer. Any help and insight is appreciated.

My best answer is that V1 should decrease on a WET runway, because the accelerate stop distance decreases to a greater degree than the accelerate go distance (deceleration is a greater problem than acceleration). On runways contaminated with snow or slush, V1 should increase because the inverse is true, acceleration is hindered more than deceleration.

Does this seem accurate?

Thanks!

Any contamination (rain, snow, lion turds) reduces V1. At least I think, I'm so dumb with this stuff sometimes.

Where's Tgreyson

 

[Olympic]

V1 on a wet runway is a lower speed than a dry runway V1 .
We use RTOW in larger jets and pull the numbers from there.

 

[BobDDuck]

I've never seen V1 increase due to stuff on the runway. We drop our V1 pretty heavily with any sort of contamination or less than ideal braking action reports. I've always assumed that this was because it will take longer to stop if you reject at V1 than on a clean runway.

Are you saying that your understanding is that you increase V1 for contamination because it take longer to get the aircraft up to speed? Isn't that kind of like arguing the airplane on a treadmill theory? While accelerating, all your potential to kinetic energy shift comes from the engines and hence surface material doesn't matter. While decelerating, some (or most, depending on the airframe) of our kinetic to potential shift is done by the friction of the brakes to the ground and hence the material matters. At least that is my understanding.

(I guess in theory if you were plowing through snow with your wheels while trying to take of it might take a slightly longer to build up speed so your accelerate go distance may stretch slightly but I've never seen that come into play with our takeoff data.)

 

[Alchemy]

Thanks BobDDuck. Yeah, I guess that was the argument I had heard, that the accelerate-go distance would be longer on certain types of contaminated (not just regular water) runways. I don't know if the friction of snow/slush/whatever acting against the undercarriage would be significant enough to necessitate an increase in v1 or not. However, like you implied, it does still seem like the degradation to braking, and thus, accelerate-stop distance, would be greater than the degradation to accelerate-go distance in those scenarios (much like the regular wet runway).

I'm not really trying to analyze....I just wanna know the right answer. If the best answer is that any sort of contamination should result in a decreased v1 I'll go with that!

 

[ppragman]

Interesting, because I know for a fact that thick snow on the ground at rotation speed will dramatically slow you down if you need to abort. That said, I doubt you'd raise V1. With 6" of snow on the ground, you're kind of shooting from the hip anyway.

 

[///AMG]

I've never used big boy V-speeds before, but we use max abort and min-go (which I believe are analagous to V1 and V2 respectively?). Looking at the charts, wet runway and lost braking action increases the runway required to stop, so that should in a practical sense, decrease this max about/decision speed. Can't speak to how folks actually utilize V1 speeds, but in my world, we would use a lower abort speed if runway conditions such as this dictated it.

 

[tgrayson]

On runways contaminated with snow or slush, V1 should increase because the inverse is true, acceleration is hindered more than deceleration.

According to "Aircraft Performance", by P.J. Swatton, runway contamination always reduces V1, but at a decreasing rate as the depth of contamination increases. He references table 4.14 in this document:

http://www.caa.co.uk/docs/33/CAP698.pdf

That document has some example tables of V1 reductions under various scenarios. For example, a 40,000 kg airplane at sea level with a 2mm depth of slush would experience a 22 knot reduction in V1. That same airplane would experience only a 13 knot reduction in V1 if the slush depth were 13mm. There are clearly two trends at work....a constant slipperiness that reduces V1 under all contamination scenarios, but an offsetting drag force that increases with the depth of contamination, without ever totally eliminating the V1 reduction. (The displayed chart shows that the lowest that the V1 reduction falls to is 0; it's possible that other airplanes might show an increase unless the JAA regulations forbid that.)

As you may note, this data has a JAA bent because the author is British. According to the book "Optimizing Jet Transport Efficiency", Carlos Padilla:

As of this writing, no regulatory agency has produced regulations dealing with the takeoff or landing performance on slippery or contaminated runways. Partially this is the result of a void in the understanding of the dynamics of an aircraft operating under these conditions, and, appropriately, the subject of much ongoing research.
​

However, this latter book was written in 1996, so it appear to be out of data with regard to the JAA, but perhaps not the FAA. That means that while the physics requires that V1 take into account the depth of contamination, it may be that FAA regulations do not require it.

 

[Alchemy]

Great answer, Tgrayson. Thanks for always coming through for us with the correct answer on tech questions.

 

[derg]

Think about it this way, if your ability to stop decreases, so does your v1 speed.

 

[dispatchguy]

Sort of.

Most heavy jets were not certified with wet runway performance. Looking in the AFM for, say a B757, and you will never find any certified data for takeoff wet runway accountability, just landing. Manufacturers have, for the most part, created advisory data -created to the same standards as certified data - but since it isnt a certification item, it's not officially required in the AFM.

Now, for example Boeing has gone back and added to their Flight Planning and Performance Manuals, the V1 deltas and max weight deltas for various wet/slippery runway conditions. Usually, those various conditions will require a downward delta V1 in order to protect the accel-stop case.

For example, on the Boeings with Paper AFMs, you can derive the true minimum V1 and Maximum V1 off the charts - and select any speed within that range as your V1. A low V1 protects the stop case, such as off a slippery runway - or when Vmcg limited (such as if the anti-skid is inop); also with the lower V1, if you do bust a motor and decide to continue, you might have a lower V2 screen height off the departure end of the runway - hence the 15ft requirement for a wet runway, versus the 35ft dry runway screen height requirement. Off of certain runways, you can theoretically lift more weight off a wet runway vs a dry runway, just due to the lower screen height requirement (why most automated systems will perform a dry weight check, to prevent this from happening).

Click here - http://www.captainpilot.com/files/BOEING PERFORMANCE/Range of V1.pdf, a Boeing flight ops engineering presentation on the acceptable Range of V1. I've met Rob Root (the author, and a Boeing Flt Ops Engineer) - VERY sharp.